This invention relates to a drill stem telemetry system, and, more particularly, to a means for transmitting data through a drill stem from the bottom of a wellbore to the surface, and vice-versa, utilizing acoustic telemetry. The need for means of transmitting downhole data to the surface during the process of a drilling operation has been recognized in the oil industry since the inception of modern drilling techniques. However, in recent years with the advent of deeper drilling operations and technical innovations which permit the detection of downhole parameters useful at the surface during a drilling operation, the need for such a telemetry system has increased, and as a result, the effort expended by the oil industry toward developing such systems has increased proportionately. An example of this need occurs when the driller needs a form of communicating from downhole to the surface, information as to the type of formation which is being drilled. Since the optimum combination of rotary speed and weight on a drill bit changes significantly with the type of formation being drilled (sand, shale, limestone, chert, etc.) a driller is unable to optimize the penetration rate without this corresponding information. Attempts have been made to develop logging while drilling systems, one such device being set forth in U.S. Pat. No. 2,755,431, but at present no system has found widespread acceptance in the industry for various reasons. Some systems have utilized cables for transmitting information from downhole to the surface but require complete withdrawal of the cable or making of connections in the cable at the surface each time a section of pipe is added. This is a cumbersome and time consuming operation and has not received acceptance. Attempts have been made to develop electrical conducting paths within a string of pipe by the use of pipe couplings which incorporate electrical conductors. Again such systems have not been developed in this country to an acceptable commercial use level. Even though the technical feasibility of such a system has been demonstrated, it requires a special drill string at greatly increased cost.
Hole deviation from the vertical and in what direction such deviation takes place is another parameter of importance in drilling operation. Such directional survey information is most important on wells which are intentionally deviated, in order to drain reservoir locations which are inaccessible or extremely costly to reach by vertical drilling. An early example of this type of drilling is the Huntington Beach and Ventura fields in California. These fields are located on the Pacific shoreline, with most of the area of the reservoir beneath the ocean. In the 30's and early 40's when these fields were drilled, it was necessary to devise the techniques and to develop tools for controlling directional drilling so that land based rigs could tap the oil beneath the ocean. The directional drilling process, then as well as now, was made more complex and expensive because of the lack of any means for telemetering this data from the bottom of the hole to the surface. As a result, such data was taken by photographic or chemical means onto instruments which were retrieved to the surface either through pulling the pipe or locating such instruments at the end of a wire line or cable which would be retrieved from the wellbore by discontinuing the drilling operation. This, of course, is a costly and time consuming operation which is aggravated in modern drilling times because of the sometimes extreme depth of wells which necessarily involves a long time factor when retrieving data by means of a wire line. Also, the high expense of operating drilling rigs, particularly in hostile environments such as offshore areas where rig time is extremely expensive becomes a very important factor since the cessation of drilling is necessary in order to retrieve data.
During the 40's, a number of companies recognized the economic potential of a telemetry system and initiated research to develop one. Most of this work was carried on by these companies independently but, invariably, after studying many of the possible transmission methods, they arrived at the same conclusion that sound transmission through the metal of the drill pipe was the most promising. Electromagnetic (radio) transmission was considered a poor second because of rapid attenuation of such signals in the formations of the earth. Since the rate of attenuation of sound in steel was known to be quite low, it was logical to assume that sound signal transmission through the metal wall of the drill pipe would be relatively simple. However, this turned out to be far from the case. In 1948 Sun Oil Co. built a system for testing the feasibility of drill pipe acoustic telemetry, which consisted of a downhole impulse sound source and a surface package designed to receive transmitted sound and measure its amplitude in each of three frequency bands. The sound source contained a battery powered motor which wound up a spring. When fully wound, the spring was released and drove a weight to deliver a sharp hammer blow to the end of the drill pipe. The receiving equipment consisted of a accelerometer attached to the drill pipe having its output connected to an amplifier which in turn fed three band pass filters for separating the energy spectrum into low, medium, and high frequency bands. The results of this feasibility study were very disappointing. The attenuation rate varied somewhat between the three bands, but was so high even in the best range as to discourage any further efforts along this line. Sun Oil concluded that acoustic telemetry was not feasible within the state of the art existing in 1948. This telemetry research project was dropped and was not reinstated until about 1969 when it was considered practical to use repeaters to overcome the high attenuation rate.
Another company doing research at that time was in the principle business of gun perforation of casing. Perforating casing is an essential step in completing oil and gas wells in which the well was drilled and cased through the producing sand as opposed to the earlier and less satisfactory practice of setting the casing just above the producing sand and drilling in for an open hole completion. This company became interested in radio active (gamma ray) logging as a means of logging cased holes, first in order to control their perforating guns more precisely, but also as a means of locating other potential producing zones behind the casing. This company established a well logging research laboratory around 1948 and one of the major projects was that of downhole telemetry. Their research program began in a very similar way to that of Sun's. After examining the alternatives, they selected drill pipe acoustic telemetry as the most promising course and set out as did Sun to measure the acoustic attenuation rate of drill pipe. The final tests in this program were convincing that drill stem acoustic telemetry was not possible. This latter test was conducted as follows: the downhole sound source consisted of a set of jars which were arranged to drop a section of drill collar about 3 feet each time the jars were actuated. On the surface, a geophone was used as the detector and was probably fed into a seismic amplifier and recorder system. The attenuation rate measured by this method was so high as to convince the experimenters that sound transmission through the drill pipe was impractical. They felt it necessary to switch their efforts to a mud pulse transmission method and to accept the greatly reduced rate of data transmission which was implied by a mud pulse system. The company continued work on the mud pulse telemetry system until the technology was sold to another party which attempted to market the system as a means of logging while drilling. In any event, the conclusion of this company, that mud pulse telemetry was the only way to go, apparently influenced much of the subsequent telemetry research so that much of the research currently taking place in the field of drill stem telemetry is centered about a technique known as mud pulse telemetry. The mud pulse system involves much more complex hardware and a slower data rate over the potentially cheaper and faster acoustic drill pipe system.
Sun Oil Co. resumed research on drill pipe acoustic telemetry in 1968, fully aware that attenuation rates would be high, but hoping to overcome this difficulty by using a number of repeater stations. Based on the attenuation measurements made in 1948, of about 12 decibels per thousand feet, it appeared feasible to use a system of repeaters spaced along the drill pipe, each receiving data from the station below at one frequency and re-transmitting at another frequency to the next station above. A transmitter and repeater system was built up to operate in this manner. In order to achieve maximum discrimination against noise, the transmission was digital and used either a single crystal controlled frequency which was turned on for one and off for zero, or in some cases a pair of closely spaced frequencies with one frequency representing a one and the other frequency a zero. Thus, the new system differed from the 1948 experiment only in that discreet frequencies were used rather than a broad band source such as the weight and spring. In order to use the multiple repeater system, three transmission frequencies were needed for the on-off logic or six for the two frequency logic. Therefore, an arbitrary selection was made. For the two frequency logic system the following pairs of frequencies were selected: 860-880 Hertz (Hz); 1060-1080 Hz, and 1260-1280 Hz. All of these frequencies were within a band for which the 1948 test indicated the attenuation rate should be in the 10-12 decibels per thousand feet range. The first field test was run using the 860-880 Hz band. This test confirmed the 10-12 decibel per thousand feet anticipated as an attenuation rate and indicated the feasibility of the repeater system as planned.
However, when it was attempted to transmit in the 1060 to 1080 Hz band, attenuation was found to be so great that no satisfactory data could be received in order to measure the exact attenuation rate. In a period of a little over a year from these first tests, a number of other frequencies were tried, but none was found to equal the 860 Hz band. It is to be remembered that there was no basis for selecting one frequency over any other, the choice being entirely random. Furthermore, it was found that the attenuation rate at the 860 Hz varied greatly from one test to another. It appeared to be dependent on the condition of the drill pipe, but in a way that was not understood. On drill pipe that was new, or in very good condition, the attenuation rate at 860 Hz was in the 10-12 decibel per thousand foot range while on badly worn drill pipe, the attenuation rate was often 30 decibels or more per thousand feet. In a search for an explanation of these results, a technical publication was studied entitled "PASSBANDS FOR ACOUSTIC TRANSMISSION IN AN IDEALIZED DRILL STRING" by Barnes and Kirkwood, published in the Journal of Acoustical Society of America, Volume 51, No. 5, (1972), pages 1606-1608. This article described a theoretical analysis of the drill pipe string as an acoustic filter and indicated that there should be a number of relatively narrow passbands separated by wider rejection bands in which no sound transmission could occur. This publication seemed to offer some explanation for the strange results of the Sun Oil tests. However, it was disappointing to find that the most successful frequency in the Sun test, i.e. 860 Hz, fell squarely in one of the rejection bands of the Barnes Kirkwood paper. Also, other frequencies that had been tried by Sun, for example 760 Hz, should have been in good transmission passbands which was contrary to the experimental data. Consequently, interest was lost in the Barnes and Kirkwood theoretical analysis and a resumption of the random choice attempts to find the three transmission bands was revived. However, this random choice technique was turning out to be a very expensive, frustrating and time consuming process.
It is readily seen from the background information above that prior attempts at acoustical telemetry in a drill pipe have met with difficulties. Therefore, it is an object of the present invention to provide an acoustic transmission system for use in a borehole, which system utilizes natural passbands within an elongated pipe string, and selecting acoustic frequencies which are adaptable to such passbands and the environment of a wellbore and more particularly the environment of a drilling operation.